Method and device for receiving downlink user data

ABSTRACT

A method for a terminal to receive downlink user data in a wireless communication system and a device supporting the method are provided. The method may comprise: a step of entering an RRC_INACTIVE state; a step of receiving, from a base station, a notification indication indicating if downlink data is transmitted in the RRC_INACTIVE state; a step of transmitting an ID of a terminal to the base station; and a step of receiving, based on the indication, a data unit from the base station in the RRC_INACTIVE state.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wireless communication system and,more particularly, to a method for a user equipment in an RRC_INACTIVEstate to receive downlink user data and a device supporting the same.

Related Art

In order to meet the demand for wireless data traffic soring since the4th generation (4G) communication system came to the market, there areongoing efforts to develop enhanced 5th generation (5G) communicationsystems or pre-5G communication systems. For the reasons, the 5Gcommunication system or pre-5G communication system is called the beyond4G network communication system or post long-term evolution (LTE)system.

In the discussion of NR standardization, an RRC_CONNECTED state and anRRC_IDLE state are defined by default as an RRC state, and anRRC_INACTIVE state is additionally introduced. A user equipment (UE) inthe RRC_INACTIVE state performs a radio control procedure similarly tothe RRC_IDLE state in order to reduce power consumption. However, the UEin the RRC_INACTIVE state maintains a connection state between the UEand a network similarly to the RRC_CONNECTED state in order to minimizea control procedure required when transitioning to the RRC_CONNECTEDstate.

SUMMARY OF THE INVENTION

Even when a user equipment (UE) is in an RRC_INACTIVE state, datatransmission and reception needs to be allowed. However, a procedure inwhich a UE in the RRC_INACTIVE state performs data transmission andreception and a procedure in which a base station (BS) of a networkcontrols data transmission and reception with a UE in the RRC_INACTIVEstate have not yet been proposed. Therefore, it is needed to propose amethod for a UE in the RRC_INACTIVE state to perform data transmissionand reception and a device supporting the same.

In accordance with one embodiment, there is provided a method in which aUE receives downlink user data in a wireless communication system. Themethod may include: entering an RRC_INACTIVE state; receiving, from aBS, a notification indication indicating whether downlink data istransmitted in the RRC_INACTIVE state; transmitting an ID of the UE tothe BS; and receiving a data unit from the BS on the basis of theindication in the RRC_INACTIVE state.

In accordance with another embodiment, there is provided a UE receivingdownlink user data in a wireless communication system. The UE mayinclude: a memory; a transceiver; and a processor to connect the memoryand the transceiver, wherein the processor may be configured to: enteran RRC_INACTIVE state; control the transceiver to receive, from a BS, anotification indication indicating whether downlink data is transmittedin the RRC_INACTIVE state; control the transceiver to transmit an ID ofthe UE to the BS; and control the transceiver to receive a data unitfrom the BS on the basis of the indication in the RRC_INACTIVE state.

It is possible to efficiently provide downlink user data to a UE in theRRC_INACTIVE STATE.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem.

FIG. 3 shows a user plane of a radio interface protocol of an LTEsystem.

FIG. 4 shows a structure of a 5G system.

FIG. 5 illustrates a procedure in which a UE in an RRC_INACTIVE statetransmits and receives data according to an embodiment of the presentinvention.

FIG. 6 illustrates a procedure in which a UE receives downlink data inthe RRC_INACTIVE state or in an RRC_ACTIVE state according to anembodiment of the present invention.

FIG. 7 illustrates a procedure in which a UE receives a data unitaccording to an embodiment of the present invention.

FIG. 8 illustrates a procedure in which a UE receives downlink user datain the RRC_INACTIVE state according to an embodiment of the presentinvention.

FIG. 9 illustrates a procedure in which a UE receives downlink user datain the RRC_ACTIVE state according to an embodiment of the presentinvention.

FIG. 10 illustrates a procedure in which a UE receives downlink userdata in the RRC_INACTIVE state according to an embodiment of the presentinvention.

FIG. 11 is a block diagram illustrating a method in which a UE receivesdownlink user data according to an embodiment of the present invention.

FIG. 12 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below may be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA maybe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA may be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA may be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is evolved from IEEE 802.16e, and provides backwardcompatibility with a system based on the IEEE 802.16e. The UTRA is apart of a universal mobile telecommunication system (UMTS). 3rdgeneration partnership project (3GPP) long term evolution (LTE) is apart of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses theOFDMA in a downlink and uses the SC-FDMA in an uplink. LTE-advanced(LTE-A) is an evolution of the LTE. 5G is an evolution of the LTE-A.

For clarity, the following description will focus on LTE-A. However,technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, etc. One eNB 20 may bedeployed per cell. There are one or more cells within the coverage ofthe eNB 20. A single cell is configured to have one of bandwidthsselected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlinkor uplink transmission services to several UEs. In this case, differentcells may be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in chargeof control plane functions, and a system architecture evolution (SAE)gateway (S-GW) which is in charge of user plane functions. The MME/S-GW30 may be positioned at the end of the network and connected to anexternal network. The MME has UE access information or UE capabilityinformation, and such information may be primarily used in UE mobilitymanagement. The S-GW is a gateway of which an endpoint is an E-UTRAN.The MME/S-GW 30 provides an end point of a session and mobilitymanagement function for the UE 10. The EPC may further include a packetdata network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which anendpoint is a PDN.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, Inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), P-GW and S-GW selection,MME selection for handovers with MME change, serving GPRS support node(SGSN) selection for handovers to 2G or 3G 3GPP access networks,roaming, authentication, bearer management functions including dedicatedbearer establishment, support for public warning system (PWS) (whichincludes earthquake and tsunami warning system (ETWS) and commercialmobile alert system (CMAS)) message transmission. The S-GW host providesassorted functions including per-user based packet filtering (by e.g.,deep packet inspection), lawful interception, UE Internet protocol (IP)address allocation, transport level packet marking in the DL, UL and DLservice level charging, gating and rate enforcement, DL rate enforcementbased on APN-AMBR. For clarity MME/S-GW 30 will be referred to hereinsimply as a “gateway,” but it is understood that this entity includesboth the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the eNB 20 are connected by means of a Uu interface. TheeNBs 20 are interconnected by means of an X2 interface. Neighbor eNBsmay have a meshed network structure that has the X2 interface. The eNBs20 are connected to the EPC by means of an S1 interface. The eNBs 20 areconnected to the MME by means of an S1-MME interface, and are connectedto the S-GW by means of S1-U interface. The S1 interface supports amany-to-many relation between the eNB 20 and the MME/S-GW.

The eNB 20 may perform functions of selection for gateway 30, routingtoward the gateway 30 during a radio resource control (RRC) activation,scheduling and transmitting of paging messages, scheduling andtransmitting of broadcast channel (BCH) information, dynamic allocationof resources to the UEs 10 in both UL and DL, configuration andprovisioning of eNB measurements, radio bearer control, radio admissioncontrol (RAC), and connection mobility control in LTE_ACTIVE state. Inthe EPC, and as noted above, gateway 30 may perform functions of pagingorigination, LTE_IDLE state management, ciphering of the user plane, SAEbearer control, and ciphering and integrity protection of NAS signaling.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem. FIG. 3 shows a user plane of a radio interface protocol of anLTE system.

Layers of a radio interface protocol between the UE and the E-UTRAN maybe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The radio interface protocol between the UE and the E-UTRAN maybe horizontally divided into a physical layer, a data link layer, and anetwork layer, and may be vertically divided into a control plane(C-plane) which is a protocol stack for control signal transmission anda user plane (U-plane) which is a protocol stack for data informationtransmission. The layers of the radio interface protocol exist in pairsat the UE and the E-UTRAN, and are in charge of data transmission of theUu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel Data istransferred between the MAC layer and the PHY layer through thetransport channel Between different PHY layers, i.e., a PHY layer of atransmitter and a PHY layer of a receiver, data is transferred throughthe physical channel using radio resources. The physical channel ismodulated using an orthogonal frequency division multiplexing (OFDM)scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physicaldownlink control channel (PDCCH) reports to a UE about resourceallocation of a paging channel (PCH) and a downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto the DL-SCH. The PDCCH may carry a UL grant for reporting to the UEabout resource allocation of UL transmission. A physical control formatindicator channel (PCFICH) reports the number of OFDM symbols used forPDCCHs to the UE, and is transmitted in every subframe. A physicalhybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement(ACK)/non-acknowledgement (NACK) signal in response to UL transmission.A physical uplink control channel (PUCCH) carries UL control informationsuch as HARQ ACK/NACK for DL transmission, scheduling request, and CQI.A physical uplink shared channel (PUSCH) carries a UL-uplink sharedchannel (SCH).

A physical channel consists of a plurality of subframes in time domainand a plurality of subcarriers in frequency domain. One subframeconsists of a plurality of symbols in the time domain. One subframeconsists of a plurality of resource blocks (RBs). One RB consists of aplurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use specific subcarriers of specific symbols of acorresponding subframe for a PDCCH. For example, a first symbol of thesubframe may be used for the PDCCH. The PDCCH carries dynamic allocatedresources, such as a physical resource block (PRB) and modulation andcoding scheme (MCS). A transmission time interval (TTI) which is a unittime for data transmission may be equal to a length of one subframe. Thelength of one subframe may be 1 ms.

The transport channel is classified into a common transport channel anda dedicated transport channel according to whether the channel is sharedor not. A DL transport channel for transmitting data from the network tothe UE includes a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting a paging message, aDL-SCH for transmitting user traffic or control signals, etc. The DL-SCHsupports HARQ, dynamic link adaptation by varying the modulation, codingand transmit power, and both dynamic and semi-static resourceallocation. The DL-SCH also may enable broadcast in the entire cell andthe use of beamforming. The system information carries one or moresystem information blocks. All system information blocks may betransmitted with the same periodicity. Traffic or control signals of amultimedia broadcast/multicast service (MBMS) may be transmitted throughthe DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the networkincludes a random access channel (RACH) for transmitting an initialcontrol message, a UL-SCH for transmitting user traffic or controlsignals, etc. The UL-SCH supports HARQ and dynamic link adaptation byvarying the transmit power and potentially modulation and coding. TheUL-SCH also may enable the use of beamforming. The RACH is normally usedfor initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to aradio link control (RLC) layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides a function ofmapping multiple logical channels to multiple transport channels. TheMAC layer also provides a function of logical channel multiplexing bymapping multiple logical channels to a single transport channel A MACsublayer provides data transfer services on logical channels.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer. The logicalchannels are located above the transport channel, and are mapped to thetransport channels.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting MBMS control information from the networkto a UE. The DCCH is a point-to-point bi-directional channel used by UEshaving an RRC connection that transmits dedicated control informationbetween a UE and the network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and may exist in both uplink and downlink. The MTCH isa point-to-multipoint downlink channel for transmitting traffic datafrom the network to the UE.

Uplink connections between logical channels and transport channelsinclude the DCCH that may be mapped to the UL-SCH, the DTCH that may bemapped to the UL-SCH and the CCCH that may be mapped to the UL-SCH.Downlink connections between logical channels and transport channelsinclude the BCCH that may be mapped to the BCH or DL-SCH, the PCCH thatmay be mapped to the PCH, the DCCH that may be mapped to the DL-SCH, andthe DTCH that may be mapped to the DL-SCH, the MCCH that may be mappedto the MCH, and the MTCH that may be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function ofadjusting a size of data, so as to be suitable for a lower layer totransmit the data, by concatenating and segmenting the data receivedfrom an upper layer in a radio section. In addition, to ensure a varietyof quality of service (QoS) required by a radio bearer (RB), the RLClayer provides three operation modes, i.e., a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM). The AM RLCprovides a retransmission function through an automatic repeat request(ARQ) for reliable data transmission. Meanwhile, a function of the RLClayer may be implemented with a functional block inside the MAC layer.In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. ThePDCP layer provides a function of header compression function thatreduces unnecessary control information such that data being transmittedby employing IP packets, such as IPv4 or IPv6, may be efficientlytransmitted over a radio interface that has a relatively smallbandwidth. The header compression increases transmission efficiency inthe radio section by transmitting only necessary information in a headerof the data. In addition, the PDCP layer provides a function ofsecurity. The function of security includes ciphering which preventsinspection of third parties, and integrity protection which preventsdata manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer takes a role of controlling a radioresource between the UE and the network. For this, the UE and thenetwork exchange an RRC message through the RRC layer. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to the configuration, reconfiguration, and release of RBs. AnRB is a logical path provided by the L1 and L2 for data delivery betweenthe UE and the network. That is, the RB signifies a service provided theL2 for data transmission between the UE and E-UTRAN. The configurationof the RB implies a process for specifying a radio protocol layer andchannel properties to provide a particular service and for determiningrespective detailed parameters and operations. The RB is classified intotwo types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB isused as a path for transmitting an RRC message in the control plane. TheDRB is used as a path for transmitting user data in the user plane.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

Referring to FIG. 2, the RLC and MAC layers (terminated in the eNB onthe network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid automatic repeat request (HARQ). TheRRC layer (terminated in the eNB on the network side) may performfunctions such as broadcasting, paging, RRC connection management, RBcontrol, mobility functions, and UE measurement reporting andcontrolling. The NAS control protocol (terminated in the MME of gatewayon the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

Referring to FIG. 3, the RLC and MAC layers (terminated in the eNB onthe network side) may perform the same functions for the control plane.The PDCP layer (terminated in the eNB on the network side) may performthe user plane functions such as header compression, integrityprotection, and ciphering.

Hereinafter, an RRC state of a UE and an RRC connection procedure aredescribed.

An RRC state indicates whether an RRC layer of the UE is logicallyconnected to an RRC layer of the E-UTRAN. The RRC state may be dividedinto two different states such as an RRC connected state and an RRC idlestate. When an RRC connection is established between the RRC layer ofthe UE and the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, andotherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has theRRC connection established with the E-UTRAN, the E-UTRAN may recognizethe existence of the UE in RRC_CONNECTED and may effectively control theUE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN,and a CN manages the UE in unit of a TA which is a larger area than acell. That is, only the existence of the UE in RRC_IDLE is recognized inunit of a large area, and the UE needs to transition to RRC_CONNECTED toreceive a typical mobile communication service such as voice or datacommunication.

In RRC_IDLE state, the UE may receive broadcasts of system informationand paging information while the UE specifies a discontinuous reception(DRX) configured by NAS, and the UE has been allocated an identification(ID) which uniquely identifies the UE in a tracking area and may performpublic land mobile network (PLMN) selection and cell re-selection. Also,in RRC_IDLE state, no RRC context is stored in the eNB.

In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and acontext in the E-UTRAN, such that transmitting and/or receiving datato/from the eNB becomes possible. Also, the UE may report channelquality information and feedback information to the eNB. InRRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs.Therefore, the network may transmit and/or receive data to/from UE, thenetwork may control mobility (handover and inter-radio accesstechnologies (RAT) cell change order to GSM EDGE radio access network(GERAN) with network assisted cell change (NACC)) of the UE, and thenetwork may perform cell measurements for a neighbor cell.

In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically,the UE monitors a paging signal at a specific paging occasion of everyUE specific paging DRX cycle. The paging occasion is a time intervalduring which a paging signal is transmitted. The UE has its own pagingoccasion.

A paging message is transmitted over all cells belonging to the sametracking area. If the UE moves from one TA to another TA, the UE willsend a tracking area update (TAU) message to the network to update itslocation.

When the user initially powers on the UE, the UE first searches for aproper cell and then remains in RRC_IDLE in the cell. When there is aneed to establish an RRC connection, the UE which remains in RRC_IDLEestablishes the RRC connection with the RRC of the E-UTRAN through anRRC connection procedure and then may transition to RRC_CONNECTED. TheUE which remains in RRC_IDLE may need to establish the RRC connectionwith the E-UTRAN when uplink data transmission is necessary due to auser's call attempt or the like or when there is a need to transmit aresponse message upon receiving a paging message from the E-UTRAN.

To manage mobility of the UE in the NAS layer, two states are defined,i.e., an EPS mobility management-REGISTERED (EMM-REGISTERED) state andan EMM-DEREGISTERED state. These two states apply to the UE and the MME.Initially, the UE is in the EMM-DEREGISTERED state. To access a network,the UE performs a process of registering to the network through aninitial attach procedure. If the attach procedure is successfullyperformed, the UE and the MME enter the EMM-REGISTERED state.

To manage a signaling connection between the UE and the EPC, two statesare defined, i.e., an EPS connection management (ECM)-IDLE state and anECM-CONNECTED state. These two states apply to the UE and the MME. Whenthe UE in the ECM-IDLE state establishes an RRC connection with theE-UTRAN, the UE enters the ECM-CONNECTED state. When the MME in theECM-IDLE state establishes an S1 connection with the E-UTRAN, the MMEenters the ECM-CONNECTED state. When the UE is in the ECM-IDLE state,the E-UTRAN does not have context information of the UE. Therefore, theUE in the ECM-IDLE state performs a UE-based mobility related proceduresuch as cell selection or reselection without having to receive acommand of the network. On the other hand, when the UE is in theECM-CONNECTED state, mobility of the UE is managed by the command of thenetwork. If a location of the UE in the ECM-IDLE state becomes differentfrom a location known to the network, the UE reports the location of theUE to the network through a tracking area update procedure.

Hereinafter, a 5G network structure is described.

FIG. 4 shows a structure of a 5G system.

In case of an evolved packet core (EPC) having a core network structureof the existing evolved packet system (EPS), a function, a referencepoint, a protocol, or the like is defined for each entity such as amobility management entity (MME), a serving gateway (S-GW), a packetdata network gateway (P-GW), or the like.

On the other hand, in case of a 5G core network (or a NextGen corenetwork), a function, a reference point, a protocol, or the like isdefined for each network function (NF). That is, in the 5G core network,the function, the reference point, the protocol, or the like is notdefined for each entity.

Referring to FIG. 4, the 5G system structure includes at least one UE10, a next generation-radio access network (NG-RAN), and a nextgeneration core (NGC).

The NG-RAN may include at least one gNB 40, and a plurality of UEs maybe present in one cell. The gNB 40 provides the UE with end points ofthe control plane and the user plane. The gNB 40 is generally a fixedstation that communicates with the UE 10 and may be referred to asanother terminology, such as a base station (BS), a base transceiversystem (BTS), an access point, or the like. One gNB 40 may be arrangedin every cell. At least one cell may be present in a coverage of the gNB40.

The NGC may include an access and mobility function (AMF) and a sessionmanagement function (SMF) which are responsible for a function of acontrol plane. The AMF may be responsible for a mobility managementfunction, and the SMF may be responsible for a session managementfunction. The NGC may include a user plane function (UPF) which isresponsible for a function of a user plane.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the gNB 40 may be connected by means of a Uu interface.The gNBs 40 may be interconnected by means of an X2 interface.Neighboring gNBs 40 may have a meshed network structure based on an Xninterface. The gNBs 40 may be connected to an NGC by means of an NGinterface. The gNBs 40 may be connected to an AMF by means of an NG-Cinterface, and may be connected to a UPF by means of an NG-U interface.The NG interface supports a many-to-many-relation between the gNB 40 andthe AMF/UPF 50.

A gNB host may perform functions such as functions for radio resourcemanagement, IP header compression and encryption of user data stream,selection of an AMF at UE attachment when no routing to an AMF can bedetermined from the information provided by the UE, routing of userplane data towards UPF(s), scheduling and transmission of pagingmessages (originated from the AMF), scheduling and transmission ofsystem broadcast information (originated from the AMF or O&M), ormeasurement and measurement reporting configuration for mobility andscheduling.

An access and mobility function (AMF) host may perform primary functionssuch as NAS signalling termination, NAS signalling security, AS securitycontrol, inter CN node signalling for mobility between 3GPP accessnetworks, idle mode UE reachability (including control and execution ofpaging retransmission), tracking area list management (for UE in idleand active mode), AMF selection for handovers with AMF change, accessauthentication, or access authorization including check of roamingrights.

A user plane function (UPF) host may perform primary functions such asanchor point for Intra-/inter-RAT mobility (when applicable), externalPDU session point of interconnect to data network, packet routing &forwarding, packet inspection and user plane part of policy ruleenforcement, traffic usage reporting, uplink classifier to supportrouting traffic flows to a data network, branching point to supportmulti-homed PDU session, QoS handling for user plane, e.g. packetfiltering, gating, UL/DL rate enforcement, uplink traffic verification(SDF to QoS flow mapping), transport level packet marking in the uplinkand downlink, or downlink packet buffering and downlink datanotification triggering.

A session management function (SMF) host may perform primary functionssuch as session management, UE IP address allocation and management,selection and control of UP function, configuring traffic steering atUPF to route traffic to proper destination, controlling part of policyenforcement and QoS, or downlink data notification.

Hereinafter, an RRC_INACTIVE state of a UE is described.

In the discussion on NR standardization, an RRC_INACTIVE state has beennewly introduced in addition to the existing RRC_CONNETED state andRRC_IDLE state. The RRC_INACTIVE state is a state introduced toefficiently manage a specific UE (for example, mMTC UE). A UE in theRRC_INACTIVE state performs a radio control procedure similarly to a UEin the RRC_IDLE state in order to reduce power consumption. However, theUE in the RRC_INACTIVE state maintains a connection state between the UEand a network similarly to the RRC_CONNECTED state in order to minimizea control procedure required when transitioning to the RRC_CONNECTEDstate. In the RRC_INACTIVE state, a radio access resource is released,but wired access may be maintained. For example, in the RRC_INACTIVEstate, the radio access resource is released, but an NG interfacebetween a gNB and a NGC or an S1 interface between an eNB and an EPC maybe maintained. In the RRC_INACTIVE state, a core network recognizes thatthe UE is normally connected to a BS. On the other hand, the BS may notperform connection management for the UE in RRC_INACTIVE state.

For a UE in a lightly connected mode, an MME may maintain the S1connection of the activated UE in order to hide a state transition andmobility from a core network. That is, for a UE in the RRC_INACTIVEstate, an AMF may maintain the NG connection of the activated UE inorder to hide a state transition and mobility from a next-generationcore (NGC). In this specification, an RRC_INACTIVE state may be similarin concept to a lightly connected mode, a lightweight connected mode, ora semi-connected mode.

Even when a UE is in the RRC_INACTIVE state, data transmission andreception needs to be allowed. However, a procedure in which a UE in theRRC_INACTIVE state performs data transmission and reception and aprocedure in which a BS (e.g., a gNB of a new RAT) of a network controlsdata transmission and reception with a UE in the RRC_INACTIVE state havenot yet been proposed. Therefore, it is needed to propose a method for aUE in the RRC_INACTIVE state to perform data transmission and receptionand a device supporting the same.

FIG. 5 illustrates a procedure in which a UE in the RRC_INACTIVE statetransmits and receives data according to an embodiment of the presentinvention.

The UE may enter the RRC_INACTIVE state and may transition to anRRC_ACTIVE state. On the contrary, the UE may enter the RRC_ACTIVE stateand may transition to the RRC_INACTIVE state. In this specification, theRRC_ACTIVE state may be similar to the RRC_CONNECTED state.

Referring to FIG. 5, in step S510, the UE may perform RRC connectionsetup and a security mode command (SMC) procedure with a BS. When the UEenters the RRC_CONNECTED state from the RRC_IDLE state, the UE mayperform RRC connection establishment, AS security activation, and radiobearer setup.

In step S520, the BS may transmit an RRC connection reconfigurationmessage to the UE. The RRC connection configuration message may be usedfor radio bearer setup and transition to the RRC_INACTIVE state.

In step S530, the UE may transmit an RRC connection reconfigurationcomplete message to the BS.

In step S540, the UE may enter the RRC_INACTIVE state. When the UEtransitions to the RRC_INACTIVE state, the UE may not release a radiobearer set up by RRC connection reconfiguration. Accordingly, the UE maymaintain the radio bearer even after transitioning to the RRC_INACTIVEstate. That is, when the radio bearer is established for the UE, theestablished radio bearer may be maintained in both the UE and the BSwhile the UE is in the RRC_INACTIVE state. Furthermore, AS security maybe kept activated in the RRC_INACTIVE state. That is, when the ASsecurity is activated for the UE, the AS security may be kept activatedfor both the UE and the BS while the UE is in the RRC_INACTIVE state.Preferably, only in a case where data transmission is allowed in theRRC_INACTIVE state, the established radio bearer may be maintained inboth the UE and the BS while the UE is in the RRC_INACTIVE state.

If data transmission is not allowed in the RRC_INACTIVE state, the UEthat has transitioned to the RRC_INACTIVE state may suspend all radiobearers. For example, if data transmission is not allowed in theRRC_INACTIVE state, the UE may suspend all radio bearers upon receivingan instruction to transition to the RRC_INACTIVE state from the BS. Forexample, if data transmission is not allowed in the RRC_INACTIVE state,the UE may suspend all radio bearers upon entering the RRC_INACTIVEstate.

However, if data transmission is allowed in the RRC_INACTIVE state, theUE may not suspend some radio bearers in which data transmission isallowed in the RRC_INACTIVE state when transitioning to the RRC_INACTIVEstate. For example, the UE may not suspend some radio bearers dependenton QoS. In this case, when the BS instructs the UE to transition to theRRC_INACTIVE state, the BS may indicate which radio bearer is notsuspended in the RRC_INACTIVE state. The BS may indicate which radiobearer is used for data transmission in the RRC_INACTIVE state. That is,if data transmission is allowed in the RRC_INACTIVE state for aparticular radio bearer, the UE having entered the RRC_INACTIVE statemay not suspend the particular radio bearer in the RRC_INACTIVE state onthe basis of a configuration from the BS. On the other hand, the UE maysuspend all radio bearers other than the particular radio bearer uponentering the RRC_INACTIVE state. Therefore, data transmission on all theradio bearers other than the particular radio bearer may be allowed onlywhile the UE is in the RRC_ACTIVE state.

The BS may determine whether data transmission is allowed in theRRC_INACTIVE state for each UE. Alternatively, the BS may determinewhether data transmission is allowed in the RRC_INACTIVE state for eachradio bearer. The BS may determine whether data transmission is allowedin the RRC_INACTIVE state for each UE or each radio bearer, for example,depending on radio bearers established for the UE. When the UE entersthe RRC_INACTIVE state, the BS may indicate to the UE whether datatransmission is allowed in the RRC_INACTIVE state for a particular radiobearer.

In addition, if the UE moves from the RRC_INACTIVE state to theRRC_ACTIVE state, the UE may resume the suspended radio bearer. Forexample, if the BS instructs the UE in the RRC_INACTIVE state totransition to the RRC_INACTIVE state, the UE may resume the suspendedradio bearer.

If data transmission is allowed in the RRC_INACTIVE state, both downlinktransmission and uplink transmission may be allowed unless aunidirectional bearer is established for the UE. In downlinktransmission, since the UE periodically monitors an RAN notification inthe RRC_INACTIVE state, the RAN notification may trigger downlinktransmission in the RRC_INACTIVE state. That is, if data transmission isallowed in the RRC_INACTIVE state, the RAN notification may be used forthe UE to receive downlink data in the RRC_INACTIVE state. The RANnotification may also be referred to as an RAN paging, an RANnotification indication, or a notification indication.

The RAN notification may be used to trigger an area update in order totransition the UE in the RRC_INACTIVE state to the RRC_ACTIVE state.Therefore, the RAN notification needs to be able to indicate to the UEwhether the UE can receive downlink data in the RRC_INACTIVE state. Inaddition, the RAN notification needs to be able to instruct the UE totransition to the RRC_ACTIVE state so that the UE receives downlink datain the RRC_ACTIVE state. The RAN notification indicating that downlinkdata can be received in the RRC_INACTIVE state can indicate a schedulefor downlink transmission to the UE. Therefore, the UE in theRRC_INACTIVE state may receive downlink data according to schedulinginformation.

For example, it is assumed that the RAN notification instructs the UE totransition to the RRC_ACTIVE state so that the UE receives downlink datain the RRC_ACTIVE state. In step S550, the UE may transmit an areaupdate to the BS in order to transition to the RRC_ACTIVE state uponreceiving the RAN notification.

In step S560, the UE may receive an area update confirmation from theBS. In step S570, the UE may enter the RRC_ACTIVE state. Accordingly,the UE may resume all radio bearers, and the BS may transmit downlinkdata through any radio bearer.

FIG. 6 illustrates a procedure in which a UE receives downlink data inthe RRC_INACTIVE state or in the RRC_ACTIVE state according to anembodiment of the present invention.

Specifically, option 1 of FIG. 6 shows a procedure in which a UEreceives downlink data in the RRC_INACTIVE state, and option 2 of FIG. 6shows a procedure in which a UE receives downlink data in theRRC_INACTIVE state. An RAN notification procedure may trigger eithertransition to the RRC_ACTIVE state or data transmission in theRRC_INACTIVE state for a particular UE.

Referring to FIG. 6, when the UE is in the RRC_INACTIVE state, a BS maydetermine whether downlink data can be transmitted in the RRC_INACTIVEstate (i.e., without transition to the RRC_ACTIVE state) or downlinkdata can be transmitted after transition to the RRC_ACTIVE state. The UEin the RRC_INACTIVE state may periodically monitor an RAN notificationindication in a particular time interval. That is, the UE in theRRC_INACTIVE state may periodically monitor an RAN notificationindication on a notification occasion. The RAN notification indicationmay trigger uplink transmission along with a UE ID.

Referring to option 1 of FIG. 6, when the BS determines downlinktransmission in the RRC_INACTIVE state, the BS may transmit downlinkdata through a radio bearer. For example, when the BS determinesdownlink transmission in the RRC_INACTIVE state and data is availablefor a radio bearer not suspended, the BS may transmit downlink datathrough the radio bearer.

Referring to option 2 of FIG. 6, when the BS determines downlinktransmission in the RRC_ACTIVE state, the BS may transmit a notificationmessage indicating transition to the RRC_ACTIVE state to the UE. Uponreceiving the notification message, the UE may transmit an area updatemessage to the BS in order to transition to the RRC_ACTIVE state. Afterthe UE enters the RRC_ACTIVE state, the UE may resume all radio bearers.Accordingly, the BS may transmit downlink data through any of the radiobearers.

FIG. 7 illustrates a procedure in which a UE receives a data unitaccording to an embodiment of the present invention.

Referring to FIG. 7, in step S700, the UE in the RRC_IDLE state mayestablish an RRC connection with a BS and may enter the RRC_ACTIVEstate.

In step S710, the UE in the RRC_ACTIVE state may receive a command fromthe BS. The command may be a state transition command. In the command,the BS may indicate whether downlink data transmission is allowed in theRRC_INACTIVE state.

Whether downlink data transmission is allowed in the RRC_INACTIVE statemay be indicated for each radio bearer. Alternatively, whether downlinkdata transmission is allowed in the RRC_INACTIVE state may be indicatedfor each priority level associated with a radio bearer. For example,whether downlink data transmission is allowed may be indicated for alogical channel priority, a quality class identifier (QCI), or a ProSeper packet priority (PPPP).

The radio bearer may be only a data radio bearer established between theUE and the BS for user data. That is, the BS may indicate whetherdownlink data transmission is allowed in the RRC_INACTIVE state only foreach data radio bearer.

The radio bearer may be only a signaling radio bearer establishedbetween the UE and the BS for an RRC message delivery or an NAS messagedelivery. That is, the BS may indicate whether downlink datatransmission is allowed in the RRC_INACTIVE state only for eachsignaling radio bearer.

The radio bearer may be either a data radio bearer or a signaling radiobearer. That is, the BS may indicate whether downlink data transmissionis allowed in the RRC_INACTIVE state for either the data radio bearer orthe signaling radio bearer.

In the command, the BS may allocate a UE ID used to identify the UE atleast while the UE is in the RRC_INACTIVE state. The UE may store the UEID.

In step S720, upon receiving the command from the BS, the UE may enterthe RRC_INACTIVE state. The RRC_INACTIVE state may indicate a state inwhich cell reselection is used and a state in which UE context is storedin the BS. The UE in the RRC_INACTIVE state may periodically monitor anRAN notification indication in a particular time interval. That is, theUE in the RRC_INACTIVE state may periodically monitor an RANnotification indication on a notification occasion.

In step S730, the BS may receive downlink data from a gateway.

In step S740, when the BS receives the downlink data from the gateway,the BS may determine whether downlink data can be transmitted in theRRC_INACTIVE state or downlink data can be transmitted after transitionto the RRC_ACTIVE state. That is, the BS may determine whether downlinkdata can be transmitted in the RRC_INACTIVE state without transition tothe RRC_ACTIVE state or downlink data can be transmitted after statetransition to the RRC_ACTIVE where handover is supported.

In step S750, the BS may transmit a notification indication to the UE.The notification indication may indicate whether downlink data istransmitted in the RRC_INACTIVE state. Additionally, the notificationindication may indicate whether a downlink notification message istransmitted in the RRC_INACTIVE state. In this specification, a downlinknotification message and a notification message may be the same message.Therefore, the UE may receive the notification indication from the BS onthe notification occasion.

In step S760, upon receiving the notification indication, the UE maytransmit an uplink message to the BB. The uplink message may indicatethe UE ID or part of the UE ID.

In step S770, upon receiving the uplink message, when data is availablefor transmission through a radio bearer that is not suspended, the BSmay transmit a downlink data unit to the UE via the radio bearer. Thedata unit may include a UE ID. For example, the UE ID may be included ina tail or a header of the data unit. The data unit may be eitherdownlink user data or a notification message.

Hereinafter, a procedure in which a UE receives downlink user data inthe RRC_INACTIVE state and a procedure in which a UE receives downlinkuser data after transitioning to the RRC_ACTIVE state will be describedin detail with reference to FIG. 8 to FIG. 10.

FIG. 8 illustrates a procedure in which a UE receives downlink user datain the RRC_INACTIVE state according to an embodiment of the presentinvention.

Referring to FIG. 8, the downlink data unit transmitted by the BS instep S770 of FIG. 7 may be downlink user data. That is, in step S770,the UE in the RRC_INACTIVE state may receive the downlink user data fromthe BS via a data radio bearer. For example, when the notificationindication transmitted by the BS in step S750 of FIG. 7 indicates thatdownlink data is transmitted in the RRC_INACTIVE state, the UE in theRRC_INACTIVE state may receive downlink user data from the BS through adata radio bearer. Then, the UE may decode or decrypt the downlink userdata.

In step S810, the UE may determine whether the downlink user data issuccessfully decoded or decrypted. Alternatively, the UE may determinewhether a UE ID included in the downlink user data corresponds to a UEID stored in the UE.

In step S820, when the downlink user data is successfully decoded ordecrypted and/or the UE ID included in the downlink user datacorresponds to the UE ID stored in the UE, the UE may transmit an ACK tothe BS. Otherwise, in step S825, the UE may transmit a NACK to the BS ormay not transmit anything.

FIG. 9 illustrates a procedure in which a UE receives downlink user datain the RRC_ACTIVE state according to an embodiment of the presentinvention.

Referring to FIG. 9, the downlink data unit transmitted by the BS instep S770 of FIG. 7 may be a downlink notification message. That is, instep S770, the UE in the RRC_INACTIVE state may receive a downlinknotification message from the BS via a signaling radio bearer or an MACcontrol element.

For example, when the notification indication transmitted by the BS instep S750 of FIG. 7 indicates that downlink data is not transmitted inthe RRC_INACTIVE state, the UE in the RRC_INACTIVE state may receive adownlink notification message from the BS.

For example, when the notification indication transmitted by the BS instep S750 of FIG. 7 indicates that a downlink notification message istransmitted in the RRC_INACTIVE state, the UE in the RRC_INACTIVE statemay receive a downlink notification message from the BS.

In step S910, the UE may determine whether a UE ID included in thedownlink notification message corresponds to a UE ID stored in the UE.

In step S920, when the UE ID included in the downlink notificationmessage corresponds to the UE ID stored in the UE and the downlinknotification message does not indicate downlink transmission in theRRC_INACTIVE state, the UE may transmit an ACK to the BS. Otherwise, instep S925, the UE may transmit a NACK to the base station or may nottransmit anything. The ACK may be an RRC message used for transition tothe RRC_ACTIVE state. Alternatively, the ACK may be an RRC message usedfor area update in the RRC_INACTIVE state.

In step S930, the UE may transition to the RRC_ACTIVE state.

In step S940, the UE that has transitioned to the RRC_ACTIVE state mayreceive downlink user data from the BS through any radio bearer.

FIG. 10 illustrates a procedure in which a UE receives downlink userdata in the RRC_INACTIVE state according to an embodiment of the presentinvention.

Referring to FIG. 10, the downlink data unit transmitted by the BS instep S770 of FIG. 7 may be a downlink notification message. That is, instep S770, the UE in the RRC_INACTIVE state may receive a downlinknotification message from the BS via a signaling radio bearer or an MACcontrol element.

For example, when the notification indication transmitted by the BS instep S750 of FIG. 7 indicates that downlink data is transmitted in theRRC_INACTIVE state, the UE in the RRC_INACTIVE state may receive adownlink notification message from the BS.

For example, when the notification indication transmitted by the BS instep S750 of FIG. 7 indicates that a downlink notification message istransmitted in the RRC_INACTIVE state, the UE in the RRC_INACTIVE statemay receive a downlink notification message from the BS.

In step S1010, the UE may determine whether a UE ID included in thedownlink notification message corresponds to a UE ID stored in the UE.

In step S1020, when the UE ID included in the downlink notificationmessage corresponds to the UE ID stored in the UE and the downlinknotification message indicates downlink transmission in the RRC_INACTIVEstate, the UE may receive downlink user data in the RRC_INACTIVE stateaccording to the downlink notification message. Otherwise, in stepS1025, the UE may transmit a NACK to the base station or may nottransmit anything.

That is, the UE may receive the downlink user data according to thedownlink notification message without transitioning to the RRC_ACTIVEstate. The downlink notification message may include schedulinginformation about the downlink user data. The scheduling informationabout the downlink user data may include at least one of the time inwhich the downlink user data is transmitted, a frequency at which thedownlink user data is transmitted, a modulation and coding scheme (MCS),a resource block, the number of retransmission times, and a DRXconfiguration.

According to an embodiment of the present invention, when a BS has asmall amount of downlink user data to transmit to a UE, the BS maytransmit the downlink user data to the UE in the RRC_INACTIVE stateaccording to the procedures of FIG. 7 and FIG. 8. When the BS has alarge amount of downlink user data to transmit to the UE, the BS maytransmit the downlink user data to the UE after transitioning the UE tothe RRC_ACTIVE state according to the procedures of FIG. 7 and FIG. 9.When the BS has a medium amount of downlink user data to transmit to theUE, the BS may provide scheduling information to the UE and may thentransmit the downlink user data to the UE in the RRC_INACTIVE stateaccording to the procedures of FIG. 7 and FIG. 10. For example, whetherthe amount of data is large or small may be determined on the basis of apredefined threshold value.

Therefore, according to the embodiment of the present invention, the BSmay efficiently provide downlink user data to the UE in the RRC_INACTIVEstate.

FIG. 11 is a block diagram illustrating a method in which a UE receivesdownlink user data according to an embodiment of the present invention.

Referring to FIG. 11, in step S1110, the UE may enter the RRC_INACTIVEstate.

In step S1120, the UE may receive, from a BS, a notification indicationindicating whether downlink data is transmitted in the RRC_INACTIVEstate. The notification indicator may further indicate whether adownlink notification message is transmitted in the RRC_INACTIVE state.

In step S1130, the UE may transmit an ID of the UE to the BS.

In step S1140, the UE may receive a data unit from the BS on the basisof the indication in the RRC_INACTIVE state. The data unit may bereceived from the BS in response to the ID of the UE.

When the notification indication indicates that downlink data istransmitted in the RRC_INACTIVE state, the data unit may be downlinkuser data received over a data radio bearer (DRB) in the RRC_INACTIVEstate. In addition, when a UE ID included in the downlink user datacorresponds to the ID of the UE, the UE may transmit an ACK to the BS.

When the notification indication indicates that downlink data istransmitted in the RRC_INACTIVE state, the data unit may be a downlinknotification message received over a signaling radio bearer (SRB) or anMAC control element in the RRC_INACTIVE state. In addition, when a UE IDincluded in the downlink notification message corresponds to the ID ofthe UE and the downlink notification message indicates that downlinkuser data is transmitted in the RRC_INACTIVE state, the UE may receivedownlink user data on the basis of the downlink notification message inthe RRC_INACTIVE state. The downlink notification message may includescheduling information about the downlink user data. The schedulinginformation about the downlink user data may include at least one of thetime in which the downlink user data is transmitted, a frequency atwhich the downlink user data is transmitted, a modulation and codingscheme (MCS), a resource block, the number of retransmission times, anda DRX configuration.

When the notification indication indicates that downlink data is nottransmitted in the RRC_INACTIVE state, the data unit may be a downlinknotification message received over a signaling radio bearer (SRB) or anMAC control element in the RRC_INACTIVE state. In addition, when a UE IDincluded in the downlink notification message corresponds to the ID ofthe UE and the downlink notification message indicates that downlinkuser data is not transmitted in the RRC_INACTIVE state, the UE maytransmit an ACK to the BS. The ACK may be at least one of an RRC messageused for transition from the RRC_INACTIVE state to the RRC_ACTIVE stateor an RRC message used for area update in the RRC_INACTIVE state.Additionally, after transmitting the ACK to the BS, the UE may enter theRRC_ACTIVE state and may receive downlink user data from the BS in theRRC_ACTIVE state.

In addition, the UE may periodically monitor the notification indicationon a notification occasion.

FIG. 12 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present invention.

A BS 1200 includes a processor 1201, a memory 1202 and a transceiver1203. The memory 1202 is connected to the processor 1201, and storesvarious information for driving the processor 1201. The transceiver 1203is connected to the processor 1201, and transmits and/or receives radiosignals. The processor 1201 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the BS may beimplemented by the processor 1201.

A UE 1210 includes a processor 1211, a memory 1212 and a transceiver1213. The memory 1212 is connected to the processor 1211, and storesvarious information for driving the processor 1211. The transceiver 1213is connected to the processor 1211, and transmits and/or receives radiosignals. The processor 1211 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the UE may beimplemented by the processor 1211.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The transceiver may include abase-band circuit for processing a wireless signal. When the embodimentis implemented in software, the aforementioned methods can beimplemented with a module (i.e., process, function, etc.) for performingthe aforementioned functions. The module may be stored in the memory andmay be performed by the processor. The memory may be located inside oroutside the processor, and may be coupled to the processor by usingvarious well-known means.

Various methods based on the present specification have been describedby referring to drawings and reference numerals given in the drawings onthe basis of the aforementioned examples. Although each method describesmultiple steps or blocks in a specific order for convenience ofexplanation, the invention disclosed in the claims is not limited to theorder of the steps or blocks, and each step or block can be implementedin a different order, or can be performed simultaneously with othersteps or blocks. In addition, those ordinarily skilled in the art canknow that the invention is not limited to each of the steps or blocks,and at least one different step can be added or deleted withoutdeparting from the scope and spirit of the invention.

The aforementioned embodiment includes various examples. It should benoted that those ordinarily skilled in the art know that all possiblecombinations of examples cannot be explained, and also know that variouscombinations can be derived from the technique of the presentspecification. Therefore, the protection scope of the invention shouldbe determined by combining various examples described in the detailedexplanation, without departing from the scope of the following claims.

1. A method in which a user equipment (UE) receives downlink user datain a wireless communication system, the method comprising: entering anRRC_INACTIVE state; receiving, from a base station (BS), a notificationindication indicating whether downlink data is transmitted in theRRC_INACTIVE state; transmitting an ID of the UE to the BS; andreceiving a data unit from the BS on the basis of the indication in theRRC_INACTIVE state.
 2. The method of claim 1, wherein the data unit isreceived from the BS in response to the ID of the UE.
 3. The method ofclaim 1, wherein, when the notification indication indicates thatdownlink data is transmitted in the RRC_INACTIVE state, the data unit isdownlink user data received over a data radio bearer (DRB) in theRRC_INACTIVE state.
 4. The method of claim 3, further comprising:transmitting an acknowledgement (ACK) to the BS when a UE ID comprisedin the downlink user data corresponds to the ID of the UE.
 5. The methodof claim 1, wherein, when the notification indication indicates thatdownlink data is transmitted in the RRC_INACTIVE state, the data unit isa downlink notification message received over a signaling radio bearer(SRB) or an MAC control element in the RRC_INACTIVE state.
 6. The methodof claim 5, further comprising: receiving downlink user data on thebasis of the downlink notification message in the RRC_INACTIVE statewhen a UE ID comprised in the downlink notification message correspondsto the ID of the UE and the downlink notification message indicates thatdownlink user data is transmitted in the RRC_INACTIVE state.
 7. Themethod of claim 6, wherein the downlink notification message comprisesscheduling information about the downlink user data.
 8. The method ofclaim 7, wherein the scheduling information about the downlink user datacomprises at least one of time in which the downlink user data istransmitted, a frequency at which the downlink user data is transmitted,a modulation and coding scheme (MCS), a resource block, a number ofretransmission times, and a DRX configuration.
 9. The method of claim 1,wherein, when the notification indication indicates that downlink datais not transmitted in the RRC_INACTIVE state, the data unit is adownlink notification message received over a signaling radio bearer(SRB) or an MAC control element in the RRC_INACTIVE state.
 10. Themethod of claim 9, further comprising: transmitting an ACK to the BSwhen a UE ID comprised in the downlink notification message correspondsto the ID of the UE and the downlink notification message indicates thatdownlink user data is not transmitted in the RRC_INACTIVE state.
 11. Themethod of claim 10, wherein the ACK is at least one of an RRC messageused for transition from the RRC_INACTIVE state to an RRC_ACTIVE stateor an RRC message used for area update in the RRC_INACTIVE state. 12.The method of claim 11, further comprising: entering the RRC_ACTIVEstate after transmitting the ACK to the BS; and receiving downlink userdata from the BS in the RRC_ACTIVE state.
 13. The method of claim 1,wherein the notification indicator further indicates whether a downlinknotification message is transmitted in the RRC_INACTIVE state.
 14. Themethod of claim 1, further comprising: periodically monitoring thenotification indication on a notification occasion.
 15. A user equipment(UE) receiving downlink user data in a wireless communication system,the UE comprising: a memory; a transceiver; and a processor to connectthe memory and the transceiver, wherein the processor is configured to:enter an RRC_INACTIVE state; control the transceiver to receive, from abase station (BS), a notification indication indicating whether downlinkdata is transmitted in the RRC_INACTIVE state; control the transceiverto transmit an ID of the UE to the BS; and control the transceiver toreceive a data unit from the BS on the basis of the indication in theRRC_INACTIVE state.
 16. The method of claim 1, wherein the UEcommunicates with at least one of a mobile terminal, a network orautonomous vehicles other than the UE.